The millimeter wave bands offer orders of magnitude more spectrum than conventional wireless frequencies but, on the other hand, suffer from increased pathloss, severe channel intermittency and inability to penetrate through solid materials. To overcome these issues, future networks must establish highly directional transmission links, typically formed with high-dimensional phased arrays, to benefit from the resulting beamforming (BF) gain and recover a sustainable quality of service.

In this context, the definition of new control layer procedures is critical and is particularly challenging for initial access (IA), which allows a mobile user equipment (UE) to establish a physical link connection with a base station (BS), a necessary step to access the network.

In current LTE systems, IA is performed on omnidirectional channels, whereas beamforming or other directional transmissions can only be performed after a physical link is established. On the other hand, in order to overcome the increased isotropic pathloss experienced at higher frequencies, in next-generation 5G systems a mechanism by which the BS and the UE determine suitable initial directions of transmission must be determined. However, directionality can significantly delay the cell search and access procedures, which is a particularly sensitive issue in 5G networks, and thus motivated us to identify and study some performance trade-offs in terms of delay, coverage, and overhead.

In our works, we investigated various IA schemes and we compared the performance of these approaches in terms of both misdetection probability and discovery time, under overhead constraints and different channel conditions. Specifically, we focused on:

Exhaustive search, a brute-force sequential beam searching technique in which both users and base stations have a predefined codebook ofdirections that cover the whole angular space and are used sequentially to transmit/receive.

Iterative search, a two-stage scanning approach in which, in the first phase, the BS transmits pilots over wider sectors while, in the second phase, it refines its search within the best such sector by steering narrower beams.

Context Information-based search, in which users are informed about the geolocations of surrounding mmWave BSs through an LTE link, with the goal of reducing the angular directions to investigate in the initial access phase.

We argued that there is a trade-off between IA latency and misdetection probability: compared to exhaustive schemes, iterative techniques require less time to perform the angular search, but exhibiting higher misdetection probabilities in general, as wider beams provide reduced gains. Moreover, the misdetection probability of pure Context Information-based approaches may be higher than for an exhaustive approach if the direct path does not correspond to good channel conditions (i.e., in NLOS or multi-path scenarios), since the beam chosen by the UE may be suboptimal.